The present invention relates generally to fabrication of a semiconductor device and more particularly to a system and a method for projecting patterned illumination through a lens system with a large numeric aperture with improved contrast over an expanded depth.
Optical lithography is a well known process for fabricating structures on a semiconductor substrate. Lithography includes applying a photosensitive material known as a photoresist on the surface of a wafer that is the subject of the fabrication. Illumination is then projected through a patterned reticle to form an image on the photosensitive material.
A reticle comprises a transparent material that includes an opaque layer representing an integrated circuit pattern (either in a positive or negative) to be imaged onto the photosensitive material.
The illumination causes a chemical reaction within the photoresist such that its solubility characteristics are altered. If the photosensitive material is a photoresist known as a positive photoresist, the portions exposed to illumination become soluble to a developer solution while unexposed portions remain relatively insoluble. If the photosensitive material is a photoresist known as a negative photoresist, the exposed portions become relatively insoluble to the developer while the unexposed portions remain soluble.
After the photoresist is exposed to illumination, the substrate is washed with a developer to remove the soluble photoresist and expose the underlying substrate or previously fabricated structures (e.g. the topography) such that the integrated circuit pattern of the reticle exists in the form of insoluble photoresist over the topography.
Thereafter, the exposed topography is etched using chemical compounds selective between the insoluble photoresist and the exposed materials to be etched.
It is a well known goal to continually reduce feature size of integrated circuits. A reduction in feature size requires an improvement in resolution of the projected image. It is well known that resolution is proportional to the wavelength of illumination divided by the numerical aperture of the projection optics. More specifically, resolution at an image plane can be improved by using a shorter wavelength illumination and using projection optics with a higher numeric aperture. For example, referring to FIG. 1, a smaller pattern size (better resolution) can be achieved at the image plane using large numeric aperture projection optics as represented by curve 12 while lower resolution is achieved at the image plane using smaller numeric aperture projection optics as represented by curve 16.
A problem with large numeric aperture projection optics is that the depth of focus is worse than small numeric aperture projection optics. More specifically, at a particular variance from the image plane, the resolution of a small numeric aperture projection optic may be better than the resolution of a large numeric aperture projection optic.
For example, referring again to FIG. 1, if a variation from the image plane increases (as represented by the vertical axis 18), a small numeric aperture projection optic maintains its resolution (as represented by curve 16) better than a large numeric aperture optic maintains its resolution (as represented by curve 12). Or stated another way, the resolution of the large numeric aperture optic xe2x80x9cfalls-offxe2x80x9d or degrades more rapidly with respect to deviation from the image plane than does a small numeric aperture optic.
Depth of focus is a concern for lithography processes. Integrated circuit structures may deviate in height from the nominal surface of the substrate. As such, the photoresist onto which the reticle pattern is to be imaged is not planar, but instead has a deep topography. This results in only a portion of the photoresist being at the image plane.
The depth of the topography limits use of a large numeric aperture projection optic. While a large numeric aperture projection optic may provide an improvement in resolution at the image plane, the image resolution will be worse than a smaller numeric aperture projection optic at topographies that deviate from the image plane.
To enable use of larger numeric aperture projection optics to improve resolution at the image plane and to accommodate the resolution degradation at topographies that deviate from the image plane, a method known as focus latitude enhancement exposure (FLEX) has been developed.
FLEX requires breaking the exposure period into multiple sub-exposure periods and varying the image plane with respect to the surface topography for each sub-exposure period. More specifically, the distance between the projection optics and the surface topography is varied for each sub-exposure period. This results in the entire exposure period consisting of an aggregation of each sub-exposure period.
FLEX operates on the principal that the contrast is best within a small depth deviation from the image plane while there exists very little contrast variations at large deviations form the image plane, the aggregate of multiple sub-exposures will create contrast at large deviations from the image plane that approximate the contrast at the image plane.
A problem with FLEX is that breaking an exposure period into multiple sub exposure periods requires a significantly longer overall time period. Between each sub-exposure period, the illumination must be turned off, the distance between the projection optics and the wafer adjusted, and the illumination turned back on to initiate the next sub-exposure period.
Therefore, what is needed is a photolithography system that provides the advantages of FLEX but does not suffer the disadvantages of operating a FLEX system.
A first aspect of the present invention is to provide a projection lithography system for exposing a photo sensitive material on a surface of a semiconductor substrate that includes surface height variations between a high level and a low level.
The projection lithography system comprises an illumination source projecting illumination within a narrow wavelength band (on the order of one Pico meter in width) centered about a nominal wavelength on an optic path towards the substrate during an exposure period.
A wavelength modulation system is positioned within the optic path and comprises means for chromatically separating the narrow wavelength band into at least two sub-bands. The first sub-band is smaller than the narrow wavelength band and is centered about a first sub-band wavelength. The second sub-band is smaller than the narrow wavelength band and is centered about a second sub-band wavelength.
The wavelength modulation system also comprises means for passing each of the first sub-band and the second sub-band during distinct time periods within the exposure period.
A patterned mask is also positioned within the optic path. A lens within the optic path focuses an image of the patterned mask onto the photosensitive material. The lens has chromatic aberration characteristics that provide for: i) focusing the image of the patterned mask at the low level when the first wavelength band is passed through the wavelength modulation system; and ii) focusing the image of the patterned mask at the high level when the second wavelength band is passed through the wavelength modulation system.
More specifically, the chromatic aberration characteristic may result in a deviation of 200 nm to 500 nm in the level at which the lens focuses the image of the patterned mask as a result of a 1 pm wavelength deviation of the illumination.
The illumination source may be a laser powered by an illumination driver, the illumination drive may pulse the laser at a frequency that is a multiple of the frequency of the continuous sinusoidal function.
The wavelength modulation system may be positioned within the optic path between the illumination source and the mask. The wavelength modulation system may vary the sub-band passed between the first sub-band and the second sub-band in a continuous sinusoidal function repeating at a frequency on the order of one kilohertz.
In a first embodiment, the wavelength modulation system comprises: i) a diffraction grating that chromatically separates the narrow wavelength band into the first wavelength band and the second wavelength band; and ii) a motor rotating the diffraction grating vary the alignment of the first wavelength band and the second wavelength band with respect to an exit slit to pass a continually varying portion of the chromatically separated illumination through the exit slit.
In a second embodiment, the wavelength modulation system comprises: i) a photo elastic crystal within the optic path to chromatically separate the illumination into the first wavelength band and the second wavelength band; and ii) a piezo electric transducer secured to the crystal for propagating sound waves through the crystal in a direction oblique to the optic path. The propagating sound waves vary the alignment of the first wavelength band and the second wavelength band with respect to an exit slit such that a continually varying portion of the chromatically separated illumination passes through the exit slit.
In a third embodiment, the wavelength modulation system comprises: i) a prism within the optic path with at least one of an entry surface and an exit surface that is oblique to the optic path for chromatically separating the illumination into the first wavelength band and the second wavelength band at the exit surface; and ii) a motion driver is secured to the prism for moving the prism to vary the alignment of the first wavelength band and the second wavelength band with respect to an exit slit such that a continually varying portion of the chromatically separated illumination passes through the exit slit.
For a better understanding of the present invention, together with other and further aspects thereof, reference is made to the following description, taken in conjunction with the accompanying drawings. The scope of the invention is set forth in the appended claims.